This invention relates to a method for removing oxides of nitrogen (or nitrogen oxides) from gaseous mixtures containing such oxides of nitrogen. More particularly, this invention is concerned with a removal of the oxides of nitrogen from atmospheric air and other gaseous mixtures containing therein oxides of nitrogen consisting predominantly of nitric oxide (NO) and nitrogen dioxide (NO.sub.2) by containing such gaseous mixtures with an aqueous alkaline solution of an alkali metal manganate.
It has been too well recognized that exhaust gases from internal combustion engines, nitric acid plants, thermal power plants, and so forth contain from several hundred to several thousand ppm of nitrogen oxides consisting mainly of nitric oxide and nitrogen dioxide, which are liable to bring about public harm (or the so-called "air pollution") in the form of photo-chemical smog, etc. Among such waste gases, a waste gas containing a high concentration of nitrogen oxides and little dust such as that discharged from nitric acid plants has already been treated on a practical and industrialized scale by washing with alkali or catalytic reduction method so as to remove the nitrogen oxides, and such a method has been practically successful. In case, however, the exhaust gas contains a low concentration of nitrogen oxides consisting mainly of nitric oxide such as that discharged from several types of boilers or internal combustion engines, there has so far been no effective method of removing the nitrogen oxides from such an exhaust gas, which has resulted in a great social problem.
The inventors have, for the purpose of solving the public harm problem due to the nitrogen oxides, made a research for an absorbing agent which shows a high absorbing capability for nitric oxide as well as nitrogen dioxide, and is inexpensive and easy to obtain. As a result of the research, the present inventors have recognized that an alkali metal mangnate or a mixture of a manganese compound and an alkali producing an alkali metal manganate under the conditions of the use of the mixture which has both oxidizing and absorbing capabilities for nitric oxide and nitrogen dioxide can be an effective absorbing agent for nitrogen oxides (NO + NO.sub.2), and applied for a patent as application Ser. No. 332,792, the parent application of this application. Further, the inventors have also recognized that an alkali metal manganate in the form of an aqueous alkaline solution shows a high absorption rate for any of nitric oxide and nitrogen dioxide, and is extremely effective for the same purpose, which is the subject theme of the present invention.